Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
Hybrid powertrains generally rely upon a mechanically operatively decoupled accelerator pedal in determining the propulsion torque request from the vehicle operator, which propulsion torque may be delivered in various contributory splits from the internal combustion engine and the electric machine(s). Similarly, hybrid powertrains may provide all or a portion of braking torque by controlling regenerative operation of the transmission electric machine(s) or by controlling the electric machines in a fashion to transfer vehicle kinetic energy to the engine and dissipate that energy via engine or exhaust braking (engine retard braking) in response to operator braking requests. Hybrid powertrains, therefore, are generally responsive to both accelerator pedal and service brake pedal requests to provide output torque in accordance therewith.
Generally, it is desirable to recover as much kinetic energy from a vehicle braking event as practical by converting to electrical energy to be returned to the electrical energy storage system of the hybrid vehicle. However, even under ideal conditions, the substantial power flows represented by decelerating a massive vehicle may not be able to be accommodated by the energy storage system. Attempting to return too much energy, or returning energy at power flows in excess of the reasonable capacity of the energy storage system to accept same, may result in irreparable harm to the energy storage system. Known regenerative braking systems therefore are generally calibrated rather conservatively to avoid damage to the energy storage system. Furthermore, for other reasons, it may be desirable to limit the power flow into the energy storage system even if its capacity to accept more energy and higher power flow is not limited by such damage considerations. Therefore, even with conservative calibrations, a regenerative braking system may not provide optimum energy return and power flow to the energy storage system in accordance with other desirable objectives.
Engine retard braking has been practiced to dissipate vehicle deceleration energy in conventional powertrain equipped vehicles. Such braking is most desirable with heavy vehicles, particularly when grade descending, to significantly enhance and minimize the need for service brake application. However, engine retard braking has conventionally been employed in a substantially uncontrolled fashion at the request of the vehicle driver in accordance with actuation of the engine braking or exhaust braking mechanism and gear ratio selection. In a hybrid powertrain equipped vehicle, such non-ideal application of engine retard braking may supplant the need for regenerative braking and forgo the significant efficiency gains that might otherwise be effected by returning the dissipated engine retarding energy to the energy storage system. Furthermore, such non-ideal and unpredictable application of engine retard braking frustrates the objective of returning regenerative braking energy in a controlled fashion to the energy storage system.
Therefore, it is desirable to coordinate control of both regenerative braking and engine retard braking in a hybrid vehicle.